Fixed 'Array to string conversion' error when Cron job arguments are in a multi-dimensional array with clear indication of the fact that the arguments are in a multi-dimensional array.
Fixes for a number of HTML validation errors.

This pull request fixes the following html validation errors:
* duplicate id 'debug-bar-cron' when on plugins page
* missing <tr> in <thead>
* old style valign/width attributes not allowed

Also makes it so css will auto-zebra-stripe the table without the need for a class name in each row
Allow for single string arguments as suggested by @ethitter
Whitespace fixes.
Bump CSS version.
Use 'empty' instead of testing against 'array()'
Use esc_html as it is important to show this in the args.
Merge branch 'pr/15'